I think the most likely scenario is that the Democrats will retain control of both houses.
Looking at the Senate, it is almost impossible to fathom how the Republicans could regain control in 2010. I'd even say that it is unlikely that they will gain any seats at all. Of the 34 seats that will be contested, 15 are currently held by Democrats and 19 are currently held by Republicans. Out of those 15 Democratic seats, only a handful will even be in play, barring some kind of cataclysmic shift in political landscape. I would say that Republicans only have a reasonable shot at 4 of them, and only a plausible shot at 7. On the flip side, the Democrats might have a reasonable shot at 8 Republican seats, and a plausible shot at 12 of them. Tactically, it's just very hard to see the GOP gaining ground there, and all but inconceivable that they would gain 9 or 10 net seats. It's likely that they will struggle to hold on to filibuster power.
The House is usually more up for grabs, since all the seats are contested. If the right political winds blow, the Republicans could certainly make some gains there - but it is still hard to imagine that they would regain control.
If I had to guess though, I'd guess that the Democrats maintain, if not increase their advantages in both houses. The timing of economic cycles is likely to play perfectly in their favor. By the time the mid terms get here, we will have seen improving economic conditions and there will be enough who believe they are the result of Democratic policies. People capable of objective, critical analysis just don't represent a large enough portion of the population to overcome those who aren't.
Early prediction for 2010 - Dems get to 258 in the House and 61 in the Senate.
Of course, anything can happen between now and then.